Thermal printing process

ABSTRACT

A thermal printing process is disclosed in which an image is formed on a heat-sensitive recording sheet composed of a support having superposed thereon a heatsensitive layer which is composed of a heat-sensitive composition dispersed in a binder therefor, the process comprising contacting a thermally-heated print head with the surface of the heat-sensitive layer to thereby form the printed image thereon, wherein the improvement comprises eliminating exfoliation of the heat-sensitive layer by employing as the binder either methyl cellulose or acetyl cellulose having a melting point of not lower than 180*C.

Sttes Pate 1 1 1111 3,871,065 Iomiyama et a]. 1 Mar. 118, 1975 1 THERMALPRINTING PROCESS 3,679,410 7/1972 Vrancken et 51. 250/316 3,689,3029/1972 Kubo et al ll7/36.8

[75] Inventors: Yoshimichi Iomiyama, Osaka;

Takashi Yamahata, Yamatokohriyama; Tomoya Yasuda, Kyoto, all of JapanPrimary Examiner-Michael Sofocleous Attorney, Agent, orFirm-Fitzpatrick, Cella, Harper & Scinto [73] Assignee: General CompanyLimited,

Osaka-shi, Japan [57] ABSTRACT [22] Filed; Aug 2 972 A thermal printingprocess is disclosed in which an image is formed on a heat-sensitiverecording sheet composed of a support having superposed thereon aheatsensitive layer which is composed of a heat- [21] Appl. No.: 277,476

[52] US. C1. 117/368, 250/316 Sensitive Composition dispersed in abinder therefor,

51 1111, c1. B41111 5/18 the Process Comprising Contacting athermally-heated 5s F1611! 61 Search 117/368; 250/316 Print head withthe Surface of the heat-sensitive layer to thereby form the printedimage thereon, wherein 56 References Cited the improvement compriseseliminating exfoliation of UNITED STATES PATENTS the heat-sensitivelayer by employing as the binder ei- 3 442 682 5/1969 Fukawa 117/36 8ther methyl cellulose or acetyl cellulose having a melt- 31592 44 7/1971Vrancken et a1. 250/316 mg pomt of not lower than 3,615,423 10/1971Vrancken 250/316 3 Claims, 4 Drawing Figures 1 THERMAL PRINTING PROCESSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a thermal printing.

2. Description of the Prior Art Thermal recording processes includedirect recording and indirect recording. It is well known that thedirect method has the advantage that a visible image can be directlyproduced while the indirect method disadvantageously requires a chemicaltreatment to produce a visible image. Thermal printing is among thedirect recording processes, wherein a thermal printer is provided with aprint head having an electric heating element. Recording is effected bybringing the electric heating element into contact with a heat sensitivesheet. In comparison with conventional mechanical processes the directheat sensitive recording process is high in printing speed, and lessnoisy so that it is suitable for an output of electronic computeroperated at high speed. The utilization of these processes will increaseas the stations are popularized.

Letters provided at the print head are usually composed of a matrix ofdots, or several bars.

Conventional heat sensitive recording paper cannot be used as is forthermal printing. In conventional heat sensitive recording processes theheat is applied to the heat sensitive paper by an optical means by theprojection of light a pattern thereon. On the contrary, in thermalprinting processes a solid print head directly contacts the surface ofthe heat sensitive paper. This direct contact of the print head causessoftening of the heat sensitive coating and exfoliation of the heatsensitive coating from the support which may be paper.

The coating thus exfoliated is attached to and deposited on the printhead. This deposit disturbs the heat transfer from the print head to theheat sensitive recording paper and thereby results in unsatisfactoryprinting. Further the deposit is often formed irregularly and thereby auniform print pattern cannot be obtained. Even if conventional heatsensitive recording paper can be used for the thermal printing, itcannot be regarded as practical unless more than ten million letters canbe printed.

Heat-sensitive recording sheets proposed heretofore are those usableonly for thermal duplicating with infrared radiation, that is, so called"thermofax.

In thermal printing where the image is formed by chemical reaction, thereaction may be between two components, or one component may beconverted into a third component which is reactable with the othercomponent, or one comconent may be converted into a colored substance toform a coloring image, etc.

In such conventional heat sensitive recording sheets, the color formingmaterials have been markedly improved, but the prior art has not paidany particular attention to the binder material.

The present inventors have now found that the melting point and theadhesion property of the binder are very important in thermal printing.

SUMMARY OF THE INVENTION According to the present invention there isprovided a process for forming an image by thermal printing on aheat-sensitive recording sheet composed of a support and heat-sensitivelayer overlying the support, the heatsensitive layer being composed of abinder selected from the group consisting of methyl cellulose and acetylcellulose having a melting point of not lower than 180C and a heatsensitive composition.

An object of the present invention is to provide a thermal printingprocess which is free from exfoliation of the heat sensitive coatingfrom the support.

Another object of the present invention is to provide thermal printingprocess in which neither smudging of the print head nor irregular heattransfer of the print head occurs.

A further object of the present invention is to provide a process forpreparing a heat-sensitive recording sheet for thermal printing asmentioned above.

BRIEF DESCRIPTION OF THE DRAWING The FIGURES show diagrammaticallyenlarged views of printing heads after they have been used with aconventional heat sensitive sheet and a heat sensitive sheet of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The binder used in the presentinvention is methylcellulose or acetylcellulose having a melting pointof not lower than 180C.

The preferred binders are (l) acetylcellulose having a weight averagemolecular weight ranging from 28,000 to 58,000, a degree ofpolymerization ranging from 175 to 360, and a viscosity (ASTM, E method,20% concentration) ranging from 10 to 40 seconds, and added in an amountranging from 40 to by weight based on the total solids weight, and (2)methyl cellulose having a weight average molecular weight ranging from26,000 to 63,000, a degree of polymerization ranging from 140 to 340,viscosity a (2% aqueous solution) ranging from 8 to cps. and added in anamount ranging from 40 to 70% by weight based on the total solidsweight.

A preferable formula of a heat sensitive coating composition is as shownbelow:

7! by weight Heat sensitive color forming ingredients (for example,compound A and compound B in prior art, infra) Inhibitor Meltingmaterial Binder (acetylcellulose or methyl cellulose) Solvent Thesolvent used in the present invention may be water or an organicsolvent. Representative organic solvents are alcohols such as methylalcohol, ethyl alcohol, and isopropyl alcohol, ketones such as acetoneand methylethyl ketone, esters such as ethyl acetate, aliphatichydrocarbons such as hexane, heptane and petroleum benzine, halogencompounds such as methylene chloride and benzylchloride and aromatichydrocarbons such as benzene, toluene and xylene.

As heat sensitive compositions, conventional heat sensitive compositionsmay be used, for example, the heat sensitive compositions disclosed inU.S. Pat. No. 3,442,682 and in U.S. Ser. No. 191,773 filed Oct. 22,1971.

Further there may be used a two component heat sensitive composition inwhich one component (component A) contains an iron salt of an aliphaticacid and the other'component (component B) contains a compound selectedfrom the class of gallic acid and its derivatives. The component A isreacted with the component B in the presence of an inhibitor of unusualcolordevelopment to produce a black color which can then be utilized asa recording image. The preferred iron salts of the aliphatic acid arethe ferrous and ferric salts of aliphatic acids containing from about 8to 24 carbon atoms. These aliphatic carboxylic acid include lauric acid,caprylic acid, pelargonic acid, capric acid and the like. The inhibitorof unusual color-development includes ethylenediamine tetracetic acidand alkali metal salts thereof. The aforementioned solvents can be usedas a solvent for both of the components A and B.

FIGS. 1 and 2 are side and top views of the printing head after printingabout 20 million letters by using a conventional heat-sensitiverecording sheet, indicating that a coating composition (hatched portion)1 flaked and scratched from the support adheres on the surfaces of mesa2 and moat 3 of a printing head 4.

FIGS. 3 and 4 are top and side views of a printing head 4 after printingabout twenty million letters by using the heat-sensitive sheet of thisinvention, indicat- The above-mentioned two compositions were treated inthe same manner as in Example 1 to produce a recording sheet. Followingthe procedure of Example 1, the similar good results were obtained.

Other than acetylcellulose, methylcellulose can be used in this Example.In this case, a mixture of water and methylalcohol may be used as asolvent.

ing that the head is clear in both of the surfaces of mesa 2 and moat 3.EXAMPLE lll With the use of a conventional heat-sensitive sheet, fSuspension Component/*1 Ferric stearatc 5 parts the coating substanceadheres to the surfaces of the 1pm mesa and the moat. The coatingsubstance thus ad- Acetylceuulose 8 P" Acetone 60 parts hered producesincomplete printing and a blurred 1m- Methylalcohol 10 parts agelsopropylalcohol 10 parts I Formula for suspension component B: Thefollowing examples WlII serve to illustrate but not Lauryl ggllate 1 5parts to limit the invention. In the following examples, all .51) 1Acetylcellulosc 8 parts parts are by weight. Acetone 60 partsMethylalcohol l0 parts lsopropylalcohol l0 parts EXAMPLE I Th d Formulafor Suspension component e above-mentionedtwo compositions were treateFerric palmitate 3 parts in the same manner as in Example 1 to produce are- Behenic acid 8 parts cording sh et; i fg i i g i pans Following asimilar test printing as in Example 1, sim- Acetone 60 parts ilarly goodresults were obtained. lsopropylalcohol 10 parts We claim;Methylalcohm 1. In a thermal printing process for forming an imageFormula for suspension component B:

5-Chloro-7-iodo-8-hydroxyquinoline 3 parts EDTA 4 parts Acetylccllulose(mp. 230C) 10 parts Acetone 60 parts lsopropylalcohol 10 partsMethylalcohol 10 parts Each of the above-mentioned two compositions wasprepared in a ball-mill. The resulting suspension liquids A and B weremixed by an agitator to form a homogeneous liquid coating agent, appliedto a white paper to form a coating of 5 ,u. in thickness, and dried atroom temperature to form a heat sensitive recording sheet.

Thermal printing was carried out on the resulting recording sheet up to20 million letters using a thermal printer having a dot matrix type heatelement (contact time of i5 milli-seconds) at a surface temperature of150 180C.

Neither flaking of the coating agent from the paper, nor adhesion on theheat element were observed.

Clear and uniform printing was obtained during the entire printingprocess.

on a heat-sensitive recording sheet composed of a support havingsuperposed thereon a heat-sensitive layer which is composed of aheat-sensitive composition dis persed in a binder therefor, said processcomprising contacting a thermally-heated print head with the surface ofthe heat-sensitive layer to form a printed image thereon, theimprovement comprising eliminating exfoliation of the heat-sensitivelayer from the support by employing, as the binder, a member selectedfrom the group consisting of methyl cellulose and acetylcellulose havinga melting point of not lower than 180C.

2. The process of claim 1 wherein the binder is acetyl cellulose havinga weight average molecular weight from 28,000 to 58,000, a degree ofpolymerization from 175 to 360 and a viscosity from 10 to 40 secondsbased on ASTM E standard at 20% concentration.

3. The process ofclaim 1 wherein the binder is methyl cellulose having aweight average molecular weight from 26,000 to 63,000, a degree ofpolymerization from 140 to 340 and a viscosity from 8 to cps at 2%aqueous solution.

1. IN A THERMAL PRINTING PROCESS FOR FORMING AN IMAGE ON AHEAT-SENSITIVE RECORDING SHEET COMPOSED OF A SUPPORT HAVING SUPERPOSEDTHEREON A HEAT-SENSITIVE LAYER WHICH IS COMPOSED OF A HEAT-SENSITIVECOMPOSITION DISPERSED IN A BINDER THEREFOR, SAID PROCESS COMPRISINGCONTACTING A THERMALLY-HEATED PRINT HEAD WITH THE SURFACE OF THEHEAT-SENSITIVE LAYER TO FORM A PRINTED IMAGE THEREON, THE IMPROVEMENTCOMPRISING ELIMINATING EXFOLIATION OF THE HEAT-SENSITIVE LAYER FROM THESUPPORT BY EMPLOYING, AS THE BINDER, A MEMBER SELECTED FROM THE GROUPCONSISTING OF METHYL CELLULOSE AND ACETYLCELLULOSE HAVING A MELTINGPOINT OF NOT LOWER THAN 180*C.
 2. The process of claim 1 wherein thebinder is acetyl cellulose having a weight average molecular weight from28,000 to 58,000, a degree of polymerization from 175 to 360 and aviscosity from 10 to 40 seconds based on ASTM E standard at 20%concentration.
 3. The process of claim 1 wherein the binder is methylcellulose having a weight average molecular weight from 26,000 to63,000, a degree of polymerization from 140 to 340 and a viscosity from8 to 120 cps at 2% aqueous solution.